From Grism to IFU: Revising the Redshift and Nature of the Massive Dusty Galaxy S1 with JWST and ALMA

TL;DR

The study revises the redshift of the dusty massive galaxy S1 from the previously reported $z_{ m grism}=5.58$ to a secure $z_{ m spec}=3.2439\pm0.0002$ using JWST/NIRSpec IFU spectroscopy that reveals multiple rest-frame optical lines. It demonstrates that the earlier grism redshift was caused by contamination from a nearby source, not S1 itself, and it reports a robust $1\rm\,mm$ dust continuum detection with a half-light radius $R_{\rm e,1mm}=0.73\pm0.10$ kpc. Updated UV-to-FIR SED modeling at the revised redshift yields $M_\star\approx(3.6-4.0)\times10^{10} M_\odot$, $M_{\rm dust}\approx(1.1-1.3)\times10^{9} M_\odot$, and $M_{\rm gas}\sim10^{11} M_\odot$, with a depletion time $\tau_{\rm dep}\approx1.4$ Gyr, placing S1 on the star-formation main sequence at $z\approx3.2$ and indicating a moderate SFR of order tens to ~100 M$_\odot$ yr$^{-1}$. The results show a dust- and gas-rich, relatively extended system, not an ultra-massive, merger-driven starburst, and underscore the importance of multi-line confirmation and IFU measurements in interpreting rare high-redshift galaxies. Overall, while the ultra-massive tail is reduced, the core conclusions about rapid early galaxy growth and the role of dusty massive galaxies remain intact.

Abstract

We report a revised spectroscopic redshift for the dusty massive galaxy S1, previously inferred with an exceptionally high baryon-to-star conversion efficiency from NIRCam slitless grism data at $z_{\rm grism}=5.58$. Our new JWST/NIRSpec IFU observations reveal multiple rest-frame optical and NIR emission lines, yielding a secure spectroscopic redshift of $z_{\rm spec}=3.2439\pm0.0002$. We show that the earlier grism-based redshift resulted from contamination by a nearby galaxy whose dispersed spectral trace overlaps with S1, illustrating a known challenge of slitless spectroscopy when only a single dispersion angle and single emission feature are available. In addition, we present new ALMA 1 mm observations, which robustly detect dust emission ($S_{\rm 1mm}=0.99\pm0.03$ mJy) and show a dust half-light radius ($R_{\rm e,1mm}=0.73\pm0.10$ kpc) slightly smaller than the stellar size ($R_{\rm e, F444W} = 0.97\pm0.01$ kpc). Using the revised redshift and compiled multi-wavelength photometry, we update the UV-to-FIR SED and find that S1 is less extreme than previously inferred, yet remains a very massive (log$M_{\star}/M_{\odot}\sim10.6$), heavily obscured star-forming galaxy. The updated SED modeling reveals S1 to be a very dust- and gas-rich system with a moderate star formation rate and a long gas depletion time ($τ_{\rm dep} \sim 1.4$ Gyr), deviating from SMGs and OFGs, but more closely resembling typical massive main-sequence galaxies. We note that, although this revision reduces the number of ultra-massive galaxies reported in Xiao et al. 2024, it does not alter the main conclusions of that work. Overall, our study clarifies the nature of S1 and underscores the importance of multi-line spectroscopic confirmation, slitless observations at multiple position angles, and IFU data for robust redshift and physical characterization of rare massive galaxies in the early Universe.

Figures (6)

• Figure 1: Images and JWST/NIRSpec IFU spectrum of S1. The top panels show image stamps in the F115W, F277W, and F444W filters, together with the corresponding RGB composite. The bottom panels show the IFU cube (left; continuum) and the extracted 1D spectrum (right). The data are shown in black, with 1$\sigma$ uncertainties in green, and the best-fit model is overplotted in red. Key emission lines used to determine the spectroscopic redshift are indicated.
• Figure 2: Zoom-in spectra of [S iii]$\lambda9069$ (left), [S iii]$\lambda9531$ (middle), He i$\lambda1.083\,\mu$m and Pa$\gamma$ (right).
• Figure 3: Left: ALMA 1 mm dust continuum image of S1. The synthesized beam size (0.618$^{\prime\prime} \times$ 0.491$^{\prime\prime}$) is shown in the lower-left corner. Right: ALMA 1 mm emission contours overlaid on the JWST RGB image. Contour levels start at 3$\sigma$ and increase in steps of $\pm2\sigma$, with positive and negative contours shown as solid and dashed lines, respectively. The intrinsic dust size ($R_{\rm e, 1mm}=0.73\pm0.10$ kpc) is slightly smaller than the stellar size from JWST F444W image Xiao2024.
• Figure 4: Illustration of the slitless grism contamination affecting the original redshift estimate of S1. The cyan lines in the top panel indicate the dispersion direction of the FRESCO grism. A nearby galaxy at $z=0.891$Mignoli2005, located $\sim41.5^{\prime\prime}$ away from S1 along the dispersion axis, produces a Br$\beta$ emission feature (bottom right; see also the Br$\beta$ map inset in the top panel) whose spectral trace overlaps with that of S1 in the slitless data. This overlap led to the earlier misidentification of the H$\alpha$ line as being associated with S1 (bottom left). The bottom panels show the 2D and 1D FRESCO NIRCam/grism spectra (F444W filter) of S1 and the contaminant, respectively.
• Figure 5: CIGALE SED fitting on the multi-wavelength photometry of S1. The best-fit SED model under CSFH, DSFH, and DSFH+burst are plotted simultaneously in the top panel, but are almost indistinguishable from each other. Red and blue dashed lines display the best-fit stellar and dust components under DSFH, respectively. Fluxes and 3-$\sigma$ upper limits are shown with the red circles and triangles, respectively. The bottom panel shows the residual between the photometry and the best-fit model under DSFH. Note that the flux density at 850 $\mu$m might be boosted by the neighboring sources due to the large SCUBA-2 beam size.